Most impacting air samplers work by directing a stream of the gas being sampled, usually air, onto a sampling media for a fixed period of time. The sampling media either is itself, or contains a flat piece of glass or plastic having a sticky surface adhesive which causes solids embedded in the air stream to stick as the air passes over it. This sampling media can then later be removed and microscopically analyzed. This sampling media may or may not contain an integrated orifice, which is used to concentrate incoming air onto a specific portion of the sampling media. An assembly for holding the sampling media in place may either be an integral part of the air sampler, or may be separate, in which case it is typically connected to the air sampler via air tubing. Note that in the later case, the term “air sampler ” typically refers to the air mover, connections, controls, etc.
The accuracy and repeatability of this air sampling technique is dependent upon, among other things, the flow rate of sampled air directed onto the sampling media. The flow rate of air is usually measured in standard cubic feet or liters per minute with the word “standard” meaning equivalent to being measured at some specific temperature, barometric pressure, and humidity, for example, a temperature of 68 degrees Fahrenheit, 14.7 psi barometric pressure and 0% relative humidity. Adjustments of some kind are often needed to compensate for not being able to sample at the standard conditions.
Many current technology samplers use either a fixed or variable speed blower or pump to force the air onto the sampling media. A blower, pump, or any other apparatus to mechanically move air can be referred to as an “air mover” or “air moving arrangement.” Those samplers with fixed speed air movers are adjusted at the factory or a calibration station for a specified airflow. Those with variable speed air movers often have a mechanical or electronic user adjustment option and may be supplied with a flow meter for aiding in this manual adjustment procedure. Samplers that have no air mover adjustment capability cannot be adjusted to compensate for changes in temperature, humidity and barometric pressure. For example, if a sampler was calibrated at 15 SLPM (standard liters per minute) airflow at sea level and is then shipped to a high-altitude location, the airflow will not be 15 SLPM for the same air mover speed, at least because there are fewer molecules in the air. Samples taken with such a sampler must be mathematically manipulated to compensate for such variables. In some cases, samples are inaccurate and/or cannot be used at all if the airflow is too low because particles do not impact the media properly. Samplers with adjustable speed air movers used in conjunction with mechanical flow meters can be adjusted for these variables, but the adjustments often have to be made manually every time a sampler is moved or conditions change, and there is no convenient way to re-adjust the sampler if conditions change during sampling. Also, the flow meter may restrict the airflow, possibly complicating the adjustment in that the meter's airflow restriction must be taken into account. The amount of restriction caused by the flow meter is dependent upon its technology.
The airflow in any sampler can also vary due to mechanical changes such as obstructions being introduced, placement and orientation of the media and orifice, build-up of sample material on the media, or air mover component wear and lubrication. Obstructions in the system can include, for example, collapsed or partially collapsed hoses and dirt in the air path or in exhaust and/or intake filters. Thus, adjustment or re-calibration is required at regular intervals to ensure accuracy in the face of such changes. Adjustments must also be made for various sampling setups, such as those that use extra air tubing to connect a more remote sampling media, or to draw air from inside a wall or other structure. Finally, bearing temperature and supply voltage for the air mover may change during sampling, adversely affecting accuracy even when a careful, skilled operator has calibrated the sampler taking all of the other factors into account.